Laminated zeolite composite and method for production thereof

ABSTRACT

The laminated zeolite composite of the present invention is characterized in that it comprises a MFI membrane being constituted by a MFI type zeolite and having a SiO 2 /Al 2 O 3  (molar ratio) of 40 to 100, and a porous substrate being constituted by a MFI type zeolite and having a SiO 2 /Al 2 O 3  (molar ratio) of 20 to 400, and that the MFI membrane is formed on the porous substrate. The composite has high separation characteristic and high permeability.

TECHNICAL FIELD

[0001] The present invention relates to a laminated zeolite compositeand a method for producing the laminated zeolite composite.

BACKGROUND ART

[0002] Zeolite composite membranes obtained by forming a zeolitemembrane on a substrate have been known and have been used as a gasseparation membrane or a liquid separation membrane. With respect tosuch zeolite composite membranes, it is known that when the SiO₂/Al₂O₃(molar ratio) of the membrane (hereinafter, it means a molar ratio whensimply “SiO₂/Al₂O₃” is mentioned) varies, the interaction between thepore surface of zeolite and a molecule passing therethrough varies andthe characteristic of the membrane when used as a separation membranevaries as well.

[0003] In, for example, p. 25 of an article by Yoji Sano and YushiKawakami in Kagaku Kogyo, February 1995 is disclosed a finding on theinfluence of SiO₂/Al₂O₃ on the permeation and separation characteristicof MFI type zeolite membrane (hereinafter referred to also as “MFImembrane”) when water and alcohol are separated from each other bypervaporation using the membrane. It is known that the membrane showsstriking alcohol selectivity when the SiO₂/Al₂O₃ thereof is increased.

[0004] As a specific example of the generally used MFI membrane, therecan be mentioned a zeolite composite membrane obtained by forming a MFImembrane on an alumina substrate. With respect to such a zeolitecomposite membrane, it is known that, during the formation of the MFImembrane, aluminum in the alumina-made substrate dissolves into the MFImembrane and is taken into the skeleton of the MFI membrane and, as aresult, the MFI membrane becomes a SiO₂/Al₂O₃-reduced MFI membrane(hereinafter this membrane is expressed also as “low-silica MFImembrane”).

[0005] Also in JP-A-6-127937 are disclosed a self-supported MFI membranenot formed on any substrate (hereinafter this membrane is expressed alsoas “MFI self-supported membrane”), into which aluminum is taken andwherein the SiO₂/Al₂O₃ is reduced; and a method for production thereof.

[0006] In the zeolite composite membrane obtained by forming a MFImembrane on an alumina substrate, however, it is impossible to controlthe SiO₂/Al₂O₃ of the MFI membrane strictly and, therefore, it isdifficult to steadily synthesize a low-silica MFI membrane exhibitinguniform separation characteristic. Further, in synthesis of the MFImembrane, a structure-directing agent is added generally and it isnecessary to remove the structure-directing agent by high-temperaturecalcination; however, since the substrate alumina and the MFI membrane(zeolite) differ in thermal expansion coefficient, the MFI membrane maygenerate cracks during calcination.

[0007] In the zeolite membrane described in JP-A-6-127937, since it is aMFI self-supported membrane, there can be avoided problems such as crackgeneration in MFI membrane due to difference in thermal expansioncoefficient between substrate and zeolite. Even in the method forproduction of MFI membrane according to the above literature, however,it is described therein that the MFI membrane formed at the initialstage of synthesis has a large SiO₂/Al₂O₃ and the SiO₂/Al₂O₃ tends tobecome smaller with the growth of the membrane; therefore, a low-silicaMFI membrane having small value in SiO₂/Al₂O₃ is obtainable only whenthe thickness of the membrane is made large. Consequently, the MFImembrane obtained exhibits permeation and separation characteristic as alow-silica MFI membrane, but has a large thickness and accordingly asmall permeation factor and has a problem of low permeability.

[0008] The present invention has been made in view of such problemspossessed by the prior art, and aims at providing a laminated zeolitecomposite having high separation characteristic and high permeability,and a method for producing such a laminated zeolite composite.

DISCLOSURE OF THE INVENTION

[0009] According to the present invention, there is provided a laminatedzeolite composite characterized in that it comprises a MFI membranebeing constituted by a MFI type zeolite and having a SiO₂/Al₂O₃ (molarratio) of 40 to 100, and a porous substrate being constituted by a MFItype zeolite and having a SiO₂/Al₂O₃ (molar ratio) of 20 to 400, andthat the MFI membrane is formed on the porous substrate.

[0010] In the present invention, the MFI membrane has a thickness ofpreferably 25 μm or less. Incidentally, the SiO₂/Al₂O₃ (molar ratio) ofthe MFI membrane may gradually decrease from the side of the membranecontacting the porous substrate toward other side thereof.

[0011] The laminated zeolite composite of the present invention issuitably used for separation of butane isomers or separation of propaneand propylene.

[0012] According to the present invention, there is also provided amethod for producing a laminated zeolite composite comprising immersinga porous substrate in a silica solcontaining sol for membrane formation,and forming a MFI membrane on the porous substrate under heatingconditions; said method being characterized in that a porous substratebeing constituted by a MFI type zeolite and having a SiO₂/Al₂O₃ (molarratio) of 20 to 400 is immersed in a sol for membrane formation having aSiO₂/Al₂O₃ (molar ratio) of 40 to 150 and a Na₂O/Al₂O₃ (molar ratio) of15 or less.

[0013] In the present invention, it is preferred to form a MFI membranebeing constituted by a MFI type zeolite and having a SiO₂/Al₂O₃ (molarratio) of 40 to 100.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a schematic drawing showing a mode for carrying out thepermeation and separation test of butane isomers.

[0015]FIG. 2 is a graph wherein the separation factor of butane isomersis plotted against the SiO₂/Al₂O₃ of MFI membrane.

BEST MODE FOR CARRYING OUT THE INVENTION

[0016] Modes for carrying out the present invention are described below.However, the present invention is not restricted to the following modesand it should be construed that design change, improvement, etc. may bemade appropriately based on the ordinary knowledge of those skilled inthe art as long as there is no deviation from the gist of the presentinvention.

[0017] The first aspect of the present invention lies in a laminatedzeolite composite characterized in that it comprises a MFI membranebeing constituted by a MFI type zeolite and having a SiO₂/Al₂O₃ (molarratio) of 40 to 100, and a porous substrate being constituted by a MFItype zeolite and having a SiO₂/Al₂O₃ (molar ratio) of 20 to 400, andthat the MFI membrane is formed on the porous substrate.

[0018] In the laminated zeolite composite of the present invention, theSiO₂/Al₂O₃ of the MFI membrane may gradually decrease from the side ofthe MFI membrane contacting with the porous substrate toward other sideof the MFI membrane. Incidentally, “gradually decrease” refers to thatthe SiO₂/Al₂O₃ decreases gradually within a range of 40 to 100. Thedetails of the first aspect are described below.

[0019] The present invention has been completed based on a finding thatthere is a correlation between the SiO₂/Al₂O₃ of MFI membrane and theseparation factor of butane isomers when the MFI membrane is used. Thatis, the laminated zeolite composite of the present invention obtained byforming a MFI membrane having a SiO₂/Al₂O₃ of 40 to 100, on a poroussubstrate being composed of a MFI type zeolite and having a SiO₂/Al₂O₃of 20 to 400, has a characteristic of gas separation, for example,separation of butane isomers.

[0020] In the above-mentioned p. 25 of an article by Yoji Sano and YushiKawakami in Kagaku Kogyo, February 1995 is disclosed the influence ofthe SiO₂/Al₂O₃ of MFI membrane on the permeation and separationcharacteristic of the MFI membrane in separation of water and alcohol,i.e. components separation when the to-be-treated material is a liquid;however, no mention is made therein on the influence on the gasseparation characteristic when the to-be-treated material is a gas. Thecorrelation between the SiO₂/Al₂O₃ of MFI membrane and thecharacteristic of gas separation including separation of butane isomershas been found for the first time in the present invention.

[0021] Further, in the laminated zeolite composite of the presentinvention, since there is formed, on a porous substrate composed of azeolite, a membrane (MFI membrane) composed of the same zeolite as inthe porous substrate, there are avoided, during production or use of thecomposite, inconveniences such as crack generation in MFI membrane dueto difference in thermal expansion coefficient between the poroussubstrate and the MFI membrane, and the MFI membrane keeps its functionat a satisfactory level.

[0022] Incidentally, “SiO₂/Al₂O₃ (molar ratio)” referred to in thepresent invention is a value obtained by measurement using energydispersive spectroscopy (EDS).

[0023] A MFI membrane having a SiO₂/Al₂O₃ of below 40 is not preferredbecause the MFI membrane tends to generate cracks on the surface.Meanwhile, a MFI membrane having a SiO₂/Al₂O₃ of beyond 100 is notpreferred because the MFI membrane, when used as a separation membrane,hardly exhibits its unique gas separation characteristic. The SiO₂/Al₂O₃of the MFI membrane is preferably 45 to 90, more preferably 50 to 80from the standpoint of superior performance as a separation membrane. Inorder to allow the MFI membrane to have a SiO₂/Al₂O₃ of 40 to 100, theSiO₂/Al₂O₃ of the porous substrate is preferably 20 to 400. TheSiO₂/Al₂O₃ of the porous substrate is more preferably 40 to 100 which isthe same level as in the MFI membrane.

[0024] In the laminated zeolite composite of the present invention, theMFI membrane preferably has a thin thickness of 25 μm or less, wherebythe MFI membrane has high separation characteristic and excellentpermeability. In order for the MFI membrane to exhibit particularlyexcellent permeability, the thickness of the MFI membrane is preferably17 μm or less, more preferably 13 μm or less. In the present invention,there is no restriction as to the lower limit of the thickness of theMFI membrane; however, the lower limit is sufficient at 0.1 μm or morein view of, for example, the function the MFI membrane when used as aseparation membrane and the practical producibility of the MFI membrane.

[0025] As the shape of the laminated zeolite composite of the presentinvention, there can be mentioned, for example, a bar shape, a pelletshape, a flat sheet shape, a tube shape, a monolithic shape and ahoneycomb shape.

[0026] The laminated zeolite composite of the present invention, byutilizing its advantages of excellent separation characteristic andpermeability and the property of hardly generating cracks, etc., can besuitably used as a separation membrane for carrying out separation ofbutane isomers or separation of propane and propylene.

[0027] Next, description is made on the second aspect of the presentinvention. The second aspect of the present invention lies in a methodfor producing a laminated zeolite composite comprising immersing aporous substrate in a silica sol-containing sol for membrane formation,and forming a MFI membrane on the porous substrate under heatingconditions; and said method is characterized in that a porous substratebeing constituted by a MFI type zeolite and having a SiO₂/Al₂O₃ (molarratio) of 20 to 400 is immersed in a sol for membrane formation having aSiO₂/Al₂O₃ (molar ratio) of 40 to 150 and a Na₂O/Al₂O₃ (molar ratio) of15 or less. In the present invention, it is preferred that a MFImembrane being constituted by a MFI type zeolite and having a SiO₂/Al₂O₃of 40 to 100 is formed. The details of the second aspect are describedbelow. Incidentally, when simply “Na₂O/Al₂O₃” is mentioned hereinafter,it means a molar ratio.

[0028] When there is used a sol for membrane formation having aSiO₂/Al₂O₃ of below 40 or beyond 150, the MFI membrane obtained isunable to have a SiO₂/Al₂O₃ of 40 to 100. Even when there is used a solfor membrane formation having a SiO₂/Al₂O₃ of 40 to 150 but when the solhas a Na₂O/Al₂O₃ of beyond 15, there is formed a MFI membrane having aSiO₂/Al₂O₃ of below 40, which is not preferred.

[0029] In order for the MFI membrane formed to reliably have aSiO₂/Al₂O₃ of 40 to 100, it is preferred to use a sol for membraneformation having a SiO₂/Al₂O₃ of 50 to 130 and is more preferred to usea SiO₂/Al₂O₃ of 55 to 120. It is also preferred to use a sol formembrane formation having a Na₂O/Al₂O₃ of 13 or less and is morepreferred to use a sol for membrane formation having a Na₂O/Al₂O₃ of 10or less. Incidentally, there is no particular restriction as to thelower limit of the Na₂O/Al₂O₃ of the sol for membrane formation used inthe present invention; however, the lower limit may be 1 or more in viewof, for example, the practical production conditions of laminatedzeolite composite.

[0030] In the present invention, a porous substrate having a SiO₂/Al₂O₃of 20 to 400 is immersed in the sol for membrane formation. As themethod for producing such a porous substrate, a known conventionalmethod may be used. In an example, tetrapropylammonium hydroxide(TPAOH), a silica sol, NaAlO₂, etc. are mixed at desired SiO₂/Al₂O₃ andTPAOH/SiO₂ (molar ratio); the resulting mixture is stirred and kneadedwith heating, to vaporize water to obtain a dry gel; the dry gel isground to obtain a powder; the powder is formed by an appropriateforming method to obtain a formed article; then, the formed article issubjected to, for example, a reaction under a steam pressure, whereby aporous substrate having a SiO₂/Al₂O₃ of desired range can be produced.Incidentally, as the appropriate forming method, there may be used anordinary ceramic forming method such as extrusion forming, CIP forming,slip casting or the like.

[0031] In the above example of the production method of poroussubstrate, it is preferred to use NaAlO₂ as a Na and Al source. When theNa₂O/Al₂O₃ is larger than 1, the porous substrate obtained hascrystalline Na separated thereon; when the Na₂O/Al₂O₃ is smaller than 1,the porous substrate obtained has a small strength. NaAlO₂ is preferredbecause it contains Na and Al at 1:1 (molar ratio) and the Na₂O/Al₂O₃can be controlled strictly at 1.

[0032] The porous substrate produced by, for example, the above methodis immersed in the sol for membrane formation having a SiO₂/Al₂O₃ of 40to 150 and a Na₂O/Al₂O₃ of 15 or less; a reaction is allowed to takeplace under a heating condition; thereby, a MFI membrane is formed onthe porous substrate. Here, “under a heating condition” refers to areaction in a temperature range of 100 to 200° C. in a pressure vessel.

[0033] The obtained membrane-formed substrate is heated appropriately toat about 500 to 600° C. in, for example, an electric oven and kept itfor about 4 to 10 hours at this temperature to remove thestructure-directing agent (TPA) used. The keeping time and the rate oftemperature increase or decrease are determined appropriately so as tomatch the sizes of membrane-formed substrate and electric oven.Incidentally, in the present invention, the thickness of the MFImembrane formed is preferably set at 25 μm or less, more preferably at17 μm or less, particularly preferably at 13 μm or less. Thereby, alaminated zeolite composite having high permeability can be obtained.

[0034] Incidentally, the thickness of the MFI membrane can be controlledby, for example, controlling the reaction time.

[0035] In the present invention, there is no restriction as to the lowerlimit of the thickness of the MFI membrane formed. However, the lowerlimit is sufficient at 0.1 μm or more in view of, for example, thefunction of MFI membrane as a separation membrane and the practicalproducibility of MFI membrane.

[0036] In the present invention, it is preferred to form a MFI membranehaving a SiO₂/Al₂O₃ of 40 to 100 because the MFI membrane can allow theresulting laminated zeolite composite to have a particularly excellentfunction as a separation membrane. In order for the MFI membrane toexhibit a particularly excellent function as a separation membrane, theMFI membrane is more preferred to have a SiO₂/Al₂O₃ ratio of 45 to 90and is particularly preferred to have a SiO₂/Al₂O₃ ratio of 50 to 80.

EXAMPLES

[0037] The present invention is specifically described below by way ofExamples. However, the present invention is not restricted to theseExamples.

Examples 1 to 2 and Comparative Examples 1 to 6

[0038] 1. Production Method of Porous Substrate A

[0039] To 16.27 g of a 10% aqueous TPAOH solution (a product of WakoPure Chemical Industries, Ltd.) were added 0.656 g of NaAlO₂ (a productof Wako Pure Chemical Industries, Ltd.) and 40.05 g of about 30 wt %silica sol (Snowtex S, a product of Nissan Chemical Industries, Ltd.).The mixture was stirred at room temperature for 1 hour using a deskshaker and then stirred and kneaded with heating at about 80° C. using ahot stirrer, to vaporize water, whereby a colorless dry gel wasobtained.

[0040] The dry gel was ground to obtain a powder, after which the powderwas subjected to uniaxial pressing with a die at a total pressure of 2ton to obtain a formed article having a disc shape having a diameter of19 mm and a thickness of 2 mm. The formed article was set on afluororesin plate in a stainless steel-made pressure vessel with afluororesin-made inner cylindrical container placing distilled water ofthe same weight as the formed article, in a state that there is nocontact between the formed article and the water. After the pressurevessel was placed in an oven of 180%, the contents of the vessel wassubjected to a reaction under a self steam pressure for 12 hours,whereby a porous substrate (porous substrate A) was obtained.

[0041] The crystal phase of the porous substrate A obtained was examinedby X-ray diffractiometry. As a result, the porous substrate A was foundto be an MFI type zeolite of perfect crystal. Incidentally, as to thecrystal phase of zeolite, a case when, in X-ray diffractiometry, therewas only a broad halo and no clear peak were found, in a region of 20 to300 (CuKα), was expressed as “amorphous”; a case when a zeolite peak wasseen even slightly, was expressed as “under crystallization”; and a casewhen all sharp peaks of zeolite were seen clearly and there was no halo,was expressed as “perfect crystal”. Incidentally, the SiO₂/Al₂O₃ of theporous substrate A was 50.

[0042] 2. Production Method of Porous Substrate B

[0043] To 16.27 g of a 10% aqueous TPAOH solution (a product of WakoPure Chemical Industries, Ltd.) was added 40.05 g of about 30 wt %silica sol (Snowtex S, a product of Nissan Chemical Industries, Ltd.).The mixture was stirred at room temperature for 1 hour using a deskshaker and then stirred and kneaded with heating at about 80° C. using ahot stirrer, to vaporize water, whereby a colorless dry gel wasobtained. The later operation was conducted in the same manner as inproduction of the porous substrate A, to obtain a porous substrate(porous substrate B).

[0044] The crystal phase of the porous substrate B obtained was examinedby X-ray diffractiometry. As a result, the porous substrate B was foundto be an MFI type zeolite of perfect crystal. Incidentally, theSiO₂/Al₂O₃ of the porous substrate B was 500 or more.

[0045] 3. Formation of MFI Membranes

[0046] There were mixed, so as to give compositions shown in Table 1,various raw materials, i.e. a 10% aqueous TPAOH solution (a product ofWako Pure Chemical Industries, Ltd.), distilled water,tetrapropylammonium bromide (a product of Wako Pure Chemical Industries,Ltd.), aluminum sulfate (14 to 18 hydrates) (a product of Wako PureChemical Industries, Ltd.), a 30 wt % silica sol (Snowtex S, a productof Nissan Chemical Industries, Ltd.) and a 4N aqueous sodium hydroxidesolution (a product of Wako Pure Chemical Industries, Ltd.). The mixturewas stirred at room temperature for 60 minutes using a desk shaker toproduce a sol for membrane formation. The sol for membrane formation wasplaced in a 100 mlstainless steel-made pressure vessel with afluororesin-made inner cylindrical container. Therein was immersed theaboveproduced porous substrate A or B. After the pressure vessel wasplaced in an oven of 180° C., the contents of the vessel was subjectedto reaction for 8 hours, whereby a MFI membrane was formed on the poroussubstrate. The resulting membraneformed substrate was placed in anelectric oven, the substrate temperature was elevated to 550° C. andthen kept it at this temperature for 4 hours to remove the TPA, wherebylaminated zeolite composites were produced (Examples 1 to 2 andComparative Examples 1 to 6).

[0047] Incidentally, the SiO₂/Al₂O₃ and Na₂O/Al₂O₃ of each sol formembrane formation and the SiO₂/Al₂O₃ of each porous substrate are shownin Table 2. TABLE 1 10% 4N aqueous tetrapropylammonium Tetrapropyl-Aluminum 30 wt % sodium- hydroxide solution Distilled ammoniumbro-sulfate silica hydroxide Porous (g) water (g) mide (g) (g) sol (g)solution (g) substrate Exam. 1 15.26 49.85 0 0.21 6 1 A Exam. 2 15.2649.85 0 0.21 6 2 A Comp. Exam. 1 15.26 49.85 1.995 0 6 0 B Comp. Exam. 215.26 49.85 0 0.21 6 1 B Comp. Exam. 3 15.26 49.85 0 0.21 6 3 B Comp.Exam. 4 10.17 49.85 1.995 0.21 6 1.6 B Comp. Exam. 5 15.26 49.85 1.995 06 0 A Comp. Exam. 6 15.26 49.85 0 0.21 6 3 A

[0048] TABLE 2 Sol for membrane formation SiO₂/Al₂O₃ Na₂O/Al₂O₃SiO₂/Al₂O₃ (molar ratio) (molar ratio) (molar ratio) of porous substrateExam. 1 40 to 150 <15 50 Exam. 2 40 to 150 <15 50 Comp. Exam. 1 >150— >500 Comp. Exam. 2 40 to 150 <15 >500 Comp. Exam. 3 40 to 150 >15 >500Comp. Exam. 4 40 to 150 <15 >500 Comp. Exam. 5 >150 — 50 Comp. Exam. 640 to 150 >15 50

[0049] (Evaluation of MFI Membranes (Separation Membranes))

[0050] 1. Thickness Measurement and Surface Observation of Each MFIMembrane

[0051] By observing the cross section and surface of each MFI membraneusing a SEM, the thickness of each MFI membrane was measured and thegeneration of cracks at the surface of each MFI membrane was examined. Acase when cracks were confirmed by the SEM observation, was rated as“present”, and a case when no cracks were confirmed, was rated as “notpresent”.

[0052] The thickness of each MFI membrane formed on each laminatedzeolite composite was 15 to 25 μm. The result of examination of thegeneration of cracks at the surface of each MFI membrane is shown inTable 3.

[0053] 3. Permeation and Separation Test

[0054] A permeation and separation test for butane isomers was carriedout by the Wicke-Kallenbach method. FIG. 1 schematically shows a modefor carrying out the permeation and separation test for butane isomers.FIG. 1 shows a state in which there is placed, inside a permeation andseparation test apparatus 10, a separated gas-holding unit 12 fittedwith a laminated zeolite composite 11. Incidentally, the permeation andseparation test apparatus 10 is heatable by an electric oven 13.

[0055] A mixed gas containing about 5% by volume of isobutane and about5% by volume of normal butane (n-butane) was fed to one side of thelaminated zeolite composite 11 using a N₂ gas as a carrier gas, under aheating condition of 200° C., and the gas after permeation, at theopposite side of the laminated zeolite composite was swept by a N₂ gasand analyzed by gas chromatography. The separation factor of butaneisomers is shown in Table 3. Incidentally, the separation factor ofbutane isomers is a value determined by the following expression (1),wherein X_(n) and X_(iso) are, respectively, the molar concentrations ofn-butane and isobutane at the supply side of the gas, and Y_(n) andY_(iso) are, respectively, the molar concentrations of n-butane andisobutane at the permeation side of the permeated gas.

Separation factor of butane isomers=(Y _(n) /Y _(iso))/(X _(n) /X_(iso))  (1) TABLE 3 Crack generation at SiO₂/Al₂O₃ Separation surfaceof MFI (molar ratio) factor of membrane of MFI membrane butane isomersExam. 1 Not present 52 84.3 Exam. 2 Not present 79 62.0 Comp. Exam. 1Not present 225 16.0 Comp. Exam. 2 Not present 130 16.3 Comp. Exam. 3Present 26 19.8 Comp. Exam. 4 Not present 107 14.2 Comp. Exam. 5 Notpresent 201 11.7 Comp. Exam. 6 Present 29 14.2

[0056] In FIG. 3 is shown a graph in which the separation factor butaneisomers are plotted against the SiO₂/Al₂O₃ value of each MFI membrane.

[0057] (Discussion)

[0058] In order to produce a laminated zeolite composite exhibitingexcellent separation characteristic, it is required that (1) the poroussubstrate used has a SiO₂/Al₂O₃ of 20 to 400, (2) the sol used formembrane formation has a SiO₂/Al₂O₃ of 40 to 150, and (3) the sol usedfor membrane formation has a NaO₂/Al₂O₃ of 15 or less. Explanation ismade below on each Example and each Comparative Example, based on theresults obtained above.

[0059] In Example 1, the porous substrate had a SiO₂/Al₂O₃ of 50 and thesol for membrane formation had a SiO₂/Al₂O₃ of 95 and a NaO₂/Al₂O₃ of6.3, and all of the above requirements (1) to (3) for production of alaminated zeolite composite exhibiting excellent separationcharacteristic are satisfied. In Example 2, the porous substrate had aSiO₂/Al₂O₃ of 50 and the sol for membrane formation had a SiO₂/Al₂O₃ of95 and a NaO₂/Al₂O₃ of 12.6, and all of the above requirements (1) to(3) for production of a laminated zeolite composite exhibiting excellentseparation characteristic are satisfied.

[0060] Meanwhile, in Comparative Examples 1 to 4, the porous substrate Bhaving a SiO₂/Al₂O₃ of >400 was used and the requirement (1) is notsatisfied. Further, in Comparative Example 1, a sol for membraneformation having a SiO₂/Al₂O₃ of >150 was used and the requirement (2)is not satisfied. In Comparative Example 3, a sol for membrane formationhaving a NaO₂/Al₂O₃ of 18.9 was used and the requirement (3) is notsatisfied.

[0061] In Comparative Examples 5 and 6, the porous substrate A having aSiO₂/Al₂O₃ of 50 was used and the requirement (1) is satisfied. However,in Comparative Example 5, a sol for membrane formation having aSiO₂/Al₂O₃ of >150 was used and the requirement (2) is not satisfiedand, in Comparative Example 6, a sol for membrane formation having aNaO₂/Al₂O₃ of 18.9 was used and the requirement (3) is not satisfied.

[0062] It is clear from the results of Table 3 and FIG. 1 that Examples1 and 2, compared with Comparative Examples 1 to 6, each show a veryhigh separation factor for n-butane and isobutane. That is, it is clearthat the laminated zeolite composites of Examples 1 and 2, as comparedwith those of Comparative Examples 1 to 6, each have excellentseparation characteristic and hardly generate inconveniences such ascracks on MFI membrane.

[0063] Incidentally, the laminated zeolite composites of Examples 1 and2, as compared with those of Comparative Examples 1 to 6, showed a highseparation factor of about 1.5 times, also in separation of propane andpropylene.

INDUSTRIAL APPLICABILITY

[0064] As described above, in the laminated zeolite composite of thepresent invention, the MFI membrane and the porous substrate each have aSiO₂/Al₂O₃ of given range and the MFI membrane is formed in a giventhickness on such a porous substrate. Therefore, the present laminatedzeolite composite has high separation characteristic and highpermeability and can be suitably used in, for example, separation ofbutane isomers or of propane and propylene.

[0065] According to the present method for producing a laminated zeolitecomposite, since a porous substrate having a SiO₂/Al₂O₃ of given rangeis immersed in a sol for membrane formation having a SiO₂/Al₂O₃ of givenrange, the MFI membrane formed can easily have a SiO₂/Al₂O₃ ofpredetermined range.

1. A laminated zeolite composite, characterized in that it comprises a MFI membrane being constituted by a MFI type zeolite and having a SiO₂/Al₂O₃ (molar ratio) of 40 to 100, and a porous substrate being constituted by a MFI type zeolite and having a SiO₂/Al₂O₃ (molar ratio) of 20 to 400, and that the MFI membrane is formed on the porous substrate.
 2. A laminated zeolite composite according to claim 1, wherein the MFI membrane has a thickness of 25 μm or less.
 3. A laminated zeolite composite according to claim 1, wherein the SiO₂/Al₂O₃ (molar ratio) of the MFI membrane decreases gradually from a side of the membrane contacting the porous substrate toward other side thereof.
 4. A laminated zeolite composite according to claim 1, which is used for separation of butane isomers.
 5. A laminated zeolite composite according to claim 1, which is used for separation of propane and propylene.
 6. A method for producing a laminated zeolite composite comprising immersing a porous substrate in a silica sol-containing sol for membrane formation and forming a MFI membrane on the porous substrate under heating conditions; said method being characterized in that a porous substrate being constituted by a MFI type zeolite and having a SiO₂/Al₂O₃ (molar ratio) of 20 to 400 is immersed in a sol for membrane formation having a SiO₂/Al₂O₃ (molar ratio) of 40 to 150 and a Na₂O/Al₂O₃ (molar ratio) of 15 or less.
 7. A method for producing a laminated zeolite composite according to claim 6, wherein a MFI membrane being constituted by a MFI type zeolite and having a SiO₂/Al₂O₃ (molar ratio) of 40 to 100 is formed.
 8. A laminated zeolite composite according to claim 2, wherein the SiO₂/Al₂O₃ (molar ratio) of the MFI membrane decreases gradually from a side of the membrane contacting the porous substrate toward other side thereof. 